Humphries joined the FBI in 1996 and first came to prominence in 1999 after an Algerian man was arrested by U.S. Customs agents as he tried to enter Washington state from Canada by ferry. Ahmed Ressam had white powder, chemicals and homemade timing devices in the trunk of his car.
Ressam claimed to be a French-speaker from Quebec. According to a 2002 Seattle Times story, Humphries, then the only French-speaker assigned to the FBI's Seattle office, was asked to question him. As soon as Humphries heard Ressam's accent and phrasing, he knew he wasn't from Quebec and told his colleagues to arrest him, the story said. It was later learned Ressam was part of an al-Qaida plot to blow up the Los Angeles airport on New Year's Eve 1999.
By 2010, Humphries had been assigned to the FBI's Tampa office and was its liaison to MacDill, home to the military's Central Command.
On May 19, 2010, 61-year-old Army veteran Ronald J. Bullock, who was camping at the base, got into an altercation with base security.
Officials said Bullock sped off on a motorcycle, but was stopped by other security officers and Humphries as he tried to exit a gate. They said he got off the motorcycle and came at Humphries and the officers while brandishing a knife. Humphries fired, killing Bullock. The shooting was later ruled justified.